Reading a vacuum gauge

Motor, Aug 2001 by Layne, Ken

There's nothing more basic than the fact that an engine is just a big air pump. It draws in air by creating a low-- pressure area in the intake manifold and cylinders, compresses the air, mixes in a little gasoline, lights a fire, generates heat and pressure and finally pumps out the spent exhaust. Our preoccupation today with things electronic sometimes makes us overlook old-fashioned mechanical symptoms of problems and the mechanical test equipment used to troubleshoot them. Vacuum gauges are often in this category, but the insight that a vacuum gauge can provide is as valuable today as it was 30, 40 or 50 years ago.

Remember that engine vacuum is just air pressure lower than atmospheric pressure. The starting point to evaluate engine vacuum is the intake manifold. When you connect a gauge to a tap on the intake, you're measuring manifold vacuum. Note that vacuum will vary in different areas of the engine, such as above or below the throttle valve and right at the intake and exhaust ports.

Vacuum drawn from an opening ahead of the throttle is called ported vacuum. Throttle opening affects ported vacuum opposite to the way it affects manifold vacuum. For example, at closed throttle, manifold vacuum is at its peak. But there is no significant vacuum at a port ahead of the throttle plate when the throttle is closed. Vacuum appears at such a port only when the throttle opens.

It's important to remember that manifold vacuum is used to power vehicle systems that need a steady supply of low-pressure air under all engine operating conditions. These systems include power brake boosters, a/c vacuum motors and some emissions controls.

Ported vacuum is used to control vehicle systems in relation to engine load. These include old-fashioned distributor vacuum advance diaphragms and carburetor assist devices. They also include many emissions control devices and transmission shift points. Under some engine load conditions, ported vacuum may equal manifold vacuum, but it can never exceed it.

Get Out the Gauge

Most vacuum gauges are graduated in inches of mercury (in.-Hg) and millimeters of mercury (mm-Hg). Some also show the modern metric scale of kilopascals (kPa). For comparison, 1 in.-Hg equals 25.4mm-Hg, or about 3.4 kPa. For this review, we'll stick to in.-Hg, or simply inches of vacuum.

Because engine vacuum is based on comparison with atmospheric pressure, it varies with altitude just as atmospheric (barometric) pressure does. The following table shows that as altitude increases, vacuum decreases about 1 inch for every 1000 feet above sea level.

Normal manifold vacuum at idle for an engine in good condition is about 18 to 22 in.-Hg. Manufacturers used to publish vacuum specs in service manuals, but this isn't as common as it was years ago. Still, the physics of internal combustion haven't changed in a hundred years, so the guidelines given here are a good starting point for vacuum gauge troubleshooting. Your best analysis based on vacuum readings will come from your own experience, however. As you use a vacuum gauge on different engines, you'll learn what's typical for one model compared to another. Some engines have reputations as low-vacuum motors; others are unusually higher than average. Experience is your best teacher.

Cranking Vacuum & Speed Tests

You can get a quick basic appraisal of engine condition by connecting a vacuum gauge to the manifold and a tachometer to the ignition to check vacuum and rpm at cranking speed. Warm up the engine first, then shut it down and connect your test equipment. Close the throttle and disable the ignition, or use a remote starter so the engine won't start. Crank the engine for 10 to 15 seconds and observe the vacuum and tach readings.

Note that different engines produce different cranking vacuum readings. Some carmakers publish specifications; others don't. Again, experience will be your best guide. What you're looking for, most importantly, is steady vacuum and cranking speed.

If the cranking speed is steady (about 200 rpm) and vacuum also is steady (around 5 inches), the engine most likely is in good mechanical condition. If rpm and vacuum are uneven, the cylinders aren't pumping equally. The engine probably has leakage past the valves, rings or head gasket. If the vacuum reading is pretty steady but cranking speed is not, you're probably looking at a damaged flywheel ring gear or starter. If the. cranking speed is normal or high but vacuum is low and slightly uneven, the engine probably has low compression or retarded valve timing. A jumped timing chain or belt is a common cause here.

The cranking vacuum test also can provide a quick test for PCV restrictions. Perform the test and note the average vacuum reading. Then pinch the hose to the PCV valve closed with your pliers and repeat the test. If the PCV system is clear, vacuum should increase. If it doesn't, check the PCV system closer for restrictions.

What Idle Tests Can Reveal

You can zero in on several basic mechanical problems by taking a quick look at manifold vacuum. Warm the engine to normal temperature-get it really warm-and connect your vacuum gauge. Make sure you connect to a manifold vacuum tap and not to ported vacuum. Connecting a tachometer also is a good idea.